Background: Neutrophil apoptosis is an important component in the resolution of inflammation, and evidence in adults suggests that an abnormality in this process contributes to the pathogenesis of inflammatory disorders. In adults, delayed apoptosis may contribute to neutrophil persistence, an apparrently critical antecedent to chronic inflammation. However, whether abnormalities in the apoptotic process are also contributory to neonatal inflammation is unclear. Acute neonatal lung inflammation can result in significant morbidity, and very premature neonates are at high risk for the development of chronic lung disease. Our observations in cord blood neutrophils of delayed apoptosis mediated by the Fas/Fas ligand pathway, in association with decreased activity of a key down-stream mediator, caspase-3, led us to hypothesize that delayed neutrophil apoptosis might also have biologic relevance to the pathogenesis of neonatal inflammation. Objectives: The main objective of the present proposal is to determine whether delayed neutrophil apoptosis has biologic relevance to neonatal inflammation. Specific aims are: 1) To determine the mechanism(s) underlying decreased caspase-3 activity in cord blood neutrophils; 2) To determine if surviving cord blood neutrophils retain inflammatory and cytotoxic activity; and 3) To characterize neutrophil apoptosis and Fas protein expression in a subset of term and preterm neonates. Methods: In mechanistic studies, we will compare adult and cord blood neutrophils for their expression regulatory proteins of the Fas/Fast pathway and their induced production of inflammatory and cytotoxic factors. In addition, the response of target cells (lung cell lines) to elicited neutrophil products will be assessed by cytotoxicity and inflammation assays. In clinical studies, we will study neutrophil apoptosis and Fas protein expression in premature neonates at risk for developing CLD, in post-conceptional age-matched controls, and in healthy adults. Health Relatedness: Acute inflammation in the neonatal period is associated with significant mortality and morbidity, and can involve a number of organ systems, including the lungs, intestine, and brain. In addition, its impaired resolution can lead to chronic, long-term complications and significant financial burden. Importantly, targeted therapies have had some recent success in diseases linked with apoptotic abnormalities. Thus, a definition of the mechanisms that regulate neutrophil apoptosis may be relevant tc the development of novel therapies which prevent or modulate neonatal inflammation. The studies outlined in this proposal will further our understanding of neutrophil biology in the developing human, and are critical to the design of future mechanistic studies.